In conventional ballpoint pens, ballpoint pens for allowing an outflow of various kinds of inks have been disclosed in recent years. Among those ballpoint pens, a ballpoint pen using an ink containing metal particles or having pigment fine particles whose diameter is large particularly in an ink with a shear-thinning property tends to have poor ink outflow characteristics in comparison with a ballpoint pen using an ink of normal colors such as black, red and blue. Therefore, as a measure taken against the above problems, a prior art is disclosed in Patent Document 1. The prior art is to ensure an ink flow rate by expanding the width of a plurality of ink grooves formed at equal intervals around an ink guide hole positioned in an axial center of a ballpoint pen tip.
Meanwhile, in recent years, as a measure taken against an ink drop in retractable ballpoint pens and a measure taken against a faint written trace in writing caused by vibration and impact to a penholder, a spring has been often inserted in a ballpoint pen tip to constantly bias a writing ball forward. Such a spring is usually formed by turning a tip-end part of a helical spring into a straight rod shape so that the tip is used to press a rear end of the writing ball. Such a spring has been regarded as inappropriate for use in such a case as, for example, the invention according to Patent Document 1 because the width of ink grooves is so wide that the rod part may stuck in the ink grooves.
Therefore, if such a spring is employed in a ballpoint pen tip, as shown in the invention according to Patent Document 2, it is necessary to create ink grooves up to the halfway of an ink guide hole and restrict a tip-end part of the spring by an inner diameter of the rear end part of the ink guide hole in order to prevent the spring from being stuck in the ink grooves.